Sheathing of electrical cables

ABSTRACT

An aluminum tube is extruded over a core containing electrical conductors insulated with paper which has been impregnated with an insulating composition, the extrusion being carried out while introducing additional insulating composition under pressure between the core and the aluminum tube. Subsequent to extrusion, the aluminum tube or sheath is sinusoidally corrugated with shallow corrugations falling within specified dimensional limits.

. a United States Patent 1 3,557,30 1

{72] inventor Paolo Gazzana Priaroggia, [56] References Cited Milan, Italy UNITED STATES PATENTS 31 1 1 2.817363 12/1957 Penrose l74/l02(.6)UX 1 la g r 19" 2,890,263 6/1959 Brandes et a1 l74/102(.6)UX 3] Assignec Pirm societa Per Azioni 3,287,490 1 1/1966 Wnght 174/102(.6)UX Milan Italy 2,870,792 1/1959 Penrose 174/102(.6)UX acorporafion ofltaly 3,394,400 7/1968 Lamons l74/102(.6)UX 32 Priority May 23 1967 2,995,616 8/1961 NlCOlfiS t 174/|02(.6)UX [33] Italy Primary ExaminerLewis H. Myers [31] 16,390/67 Assistant Examiner-A. T. Grirnley Attorney-Ward, McElhannon, Brooks & Fitzpatrick 1 ABSTRACT: An aluminum tube is extruded over a core con- [54] ELECTRICAL CABLES taining electrical conductors insulated with paper which has rawmg been impregnated with an insulating composition, the extru- [52] US. Cl 174/ 102, sion being carried out while introducing additional insulating 174/25 composition under pressure between the core and the alu- [51] Int. Cl H0lb 7/20 minum tube. Subsequent to extrusion, the aluminum tube or [50] Field of Search 174/102, sheath is sinusoidally corrugated with shallow corrugations falling within specified dimensional limits.

SHEATI-IING OF ELECTRICAL CABLES DISCLOSURE The present invention relates to electric cables of the type having a core, an intermediate layer or an insulating composition. and an outer metallic sheath. More particularly, the invention relates to electric cables for power transmission which have an outer metallic sheath that is corrugated and to a process for sheathing said cables.

A typical method of insulating electric power cables is to surround the conductors with layers of paper that have been impregnated with an insulating composition such as a very thick oil or an oil mixed with suitable resinous material. Heretofore, such cables have generally been covered with a protective extruded metal sheath made. for example, of lead or aluminum. Lead has been used for a long time due to its malleability and its ability of being extruded easily.

The adoption of aluminum is more recent, since the use of this material for extruded sheaths had to await the development of modern processing techniques. Aluminum is preferred to lead because of its considerably better mechanical qualities, its low specific gravity, its good resistance to vibration, and its lower electric resistance. However, aluminum sheaths, as compared with lead sheaths, have less flexibility. That is, the aluminum sheath is less able to be subjected to substantial bending without being damaged.

Heretofore, in order to improve the flexibility of the aluminum sheath, it has been corrugated. The corrugating operation has generally been carried out so as to provide a ratio between the depth and the pitch of the corrugation which is greater-than 1:7, the depth ofthe corrugations falling within a range of between 2.5 and 7.25 millimeters. As used herein, the term depth means one-half of the value of the difference between the maximum diameter and the minimum diameter of the sheath.

Providing such deep corrugations do not involve any difficulty when the core of the cables is impregnated with a liquid of low viscosity. However, if the impregnation is made with heavy oils or resin thickened oils, the corrugating operation often yields unsatisfactory results. The procedure that has been used to form a corrugated sheath is to apply it initially as a tube with a diameter larger than that of the cable core. This leaves a space between the sheath and the core which is not filled with the impregnating composition. The tube is subsequently corrugated. It has been found that zones exist near the crests of the corrugations which are not filled with the impregnating composition. Atmospheric air or gases can be trapped in such zones, and in course of time this can give rise to deleterious consequences with regard to the dielectric characteristics of the insulation. Furthermore, portions of the impregnating composition forming part of the insulation on the core can separate from the core and fall into the zones. This reduces the insulation at such points and adversely affects the ability of the cable to withstand the electric gradient when the cable is placed into use.

A proposed solution to eliminate the voids or nonimpregnated zones involves admitting the impregnating composition, after the corrugating operation, into the space between the sheath and the cable core by means of a pump or injecting device. However, this procedure does not permit complete filling of the nonimpregnated zones due to the high viscosity of the composition and the resistance it encounters in its passage along the space between the sheath and the core.

It is an object of the present invention to provide an electric cable insulated with paper impregnated with an insulating composition and provided with a corrugated metal sheath which is free from the aforementioned drawbacks. In ac cordance with the invention, there is provided an electric cable comprising a core, an intermediate layer of an insulating composition, and an outer metallic sheath, the sheath being sinusoidally corrugated with a minimum diameter at least one millimeter larger than the outside diameter of the core and with the depth of the corrugations in the range of between 0.5

and L5 millimeters and the ratio of the depth to the pitch of the corrugations in the range of 1:25 to 1:15.

As used herein. the expression -sinusoidally corrugated" refers to a corrugation which in longitudinal section has a profile whose curvature varies between constant positive and negative values.

A further object. in accordance with the present invention, is the provision of a process for fabricating electric cables of the aforementioned type, said process comprising the steps of passing the core through an extruder and extruding thereon the metal sheath while introducing the composition under pressure between the core and the sheath, and corrugating the extruded sheath with the core in situ such that the final dimensions fall within the ranges indicated above.

The invention will be better understood after reading the following detailed description of the presently preferred embodiment thereof with reference to the attached drawings in which:

FIG. 1 is a generally diagrammatic view of the equipment necessary to practice the process of the present invention; and

FIG. 2 is a fragmentary longitudinal sectional view of the resultant cable produced by the apparatus shown in FIG. 1.

In FIG 1, l is a conventional extruder for extruding aluminum sheaths and 2 is a cooling jacket intended to cause a drop in the temperature of the sheath immediately upon its leaving the extruder. Cooling of the sheath within the jacket 2 may be accomplished by means ofa water spray.

The cable core, designated 3 in the figure, can be supplied from a revolving platform, not visible in the figure, on which it is supported in coils. The core 3 enters the extruder 1 after passing through a small tank 4, connected to the entrance to the extruder and filled with impregnating composition. The tank 4 is always used during the covering of cables of the aforesaid type with a smooth sheath in order to restore, prior to entry of the core into the extruder, the quantity of impregnating composition which has dropped from the core during its stay on the revolving platform and on the way from the latter to the extruder.

Downstream of the extruder the cable core, now covered with the aluminum sheath 5, passes through a conventional corrugating device 6 which imparts to it a helical corrugation having a sinusoidal profile as best seen in FIG. 2, As stated above, by carrying out the corrugating step such that the ratio between the depth 1 and the pitch p of the corrugations lies between 1:15 and 1:25, it is possible to eliminate the voids or unfilled spaces that would otherwise develop in the regions 7 (see FIG. 2) between the sheath and the core, all without adversely affecting the flexibility of the cable. The cable remains highly flexible even though all of the regions 7 are completely filled with the insulating composition. It is believed that the empty zones or voids are eliminated due to the fact that the space 8 between the sheath 5 and the core 3, in the region between the corrugating device 6 and the extruder 1, is filled completely with impregnating composition coming from tank 4, and, therefore, the subsequent corrugation of the sheath cannot give rise to void zones but at the most, owing to the reduced enclosed volume of the corrugated sheath, only to a weak return flow of the impregnating composition.

As stated above, an object of the present invention is to provide a process for sheathing cables of the type having a core insulated with paper impregnated with an insulating composition and covered with a corrugated metal sheath. An important feature of the process is that the application of the metal sheath is carried out under a pressure head of the composition contained in the tank 4, thereby filling the entire space between the sheath and the cable core.

Heretofore, it has been conventional to pass the cable core through a tank, such as the tank 4, when covering the cable with a smooth or uncorrugated sheath. In such case, no difficulty is encountered in filling the space between the core and sheath, the pressure head of the composition readily accomplishing this purpose. But. up to the present time it has not been possible to employ such process for cables having a corrugated sheath. It has been found that using the conventional depths for the corrugations and the conventional ratios between the depth and the pitch ofthe corrugations. when the cable is sheathed under the pressure head of the composition and the sheath is corrugated, considerable distortion and in some cases rupture occurs in the latter. Moreover. the insulating papers may be easily torn, with a consequent damage to the insulating characteristics of the cores themselves It is believed that this may be due to the fact that. on account of the considerable depth of the corrugations. the initial diameter of the aluminum sheath. and therefore. the quantity of composition contained in the space 8. must be increased. The subsequent corrugation which. owing to its depth, causes about a 50 percent reduction in the volume enclosed in the space 8, gives rise to a considerable return flow of the composition. The composition is at a relatively low temperature. having been cooled during its passage through the jacket 2. and its viscosity is such that its return flow is hindered or prevented. As a consequence. the papers can be torn and distortion or rupture of the sheath can occur.

It has been found that for values of the corrugation ratio less than 1:25, the flexibility of the cable is .so reduced that it is comparable with that of cables provided with a smooth or uncorrugated sheath. ()nly if the value of said ratio falls within the above indicated range is it possible to cover the cable with an aluminum sheath under a pressure head ofinsulating composition (avoiding thereby the existence of empty spaces) and then to corrugate said sheath while obtaining acceptable flexibility values for the cable.

In an actual embodiment of the invention, a cable core, 50 millimeters in diameter, was covered with an aluminum sheath. The applicable dimensions were:

corrugation ratio 1:25;

corrugation depth 1 1 millimeter;

corrugation pitch p= 25 millimeters;

initial inner diameter of the extruded sheath 53 millimeters;

inner diameter of the sheath after corrugation 5 l millimeters at the valleys and practically 53 millimeters at the crests;

thickness s of the sheath 1 millimeter; and

minimum space between the sheath and the core 0.5 millimeter.

I It is in fact advisable to leave at least a minimum space, equal to about 0.5 millimeter, between the aluminum sheath and the cable core in order to avoid with assurance any damage to the insulating papers during the corrugating operation. during subsequent flexing and during any possible welding in the making ofjoints.

Flexure tests have been carried out on a core so prepared up to bending diameters of the order of 30 times the average diameter of the sheath, and no deformations were observed-in the sheath section. Subsequently. the sheath was removed and a perfect filling with the impregnating composition of all the spaces between the core and the sheath was found without finding any damage to the insulating papers.

The sheaths constructed in accordance with the present invention avoid thedrawbacks generally encountered heretofore in the corrugating operation. In fact. owing to the reduced extent of the corrugations, which can be produced in a simple and easy manner, a smaller quantity of aluminum is required. thereby reducing the total cost of the cable.

Having described the invention with reference to a presently preferred embodiment, it should be understood that numerous changes may be made therein as will appear evident to those skilled in the present art without departing from the true spirit of the invention as defined in the appended claims.

lclaim;

1. An electric cable comprising a core having a conductor surrounded by a layer of insulation, an outer metallic sheath surrounding said core and an insulating composition intermediate said layer and said outer metallic sheath. said sheath being sinusoidally corrugated with a minimum interior diameter at least 1 millimeter larger than the outside diameter of said layer and with thedepth of the corrugations in the range of between 0.5 and 1.5 millimeters and the ratio of said depth to the pitch of said corrugations in the range of 1:25 to 1:15.

2. An electric cable according to claim 1, wherein said sheath consists of aluminum.

3. An electric cable according to claim 1, wherein the entire space between said layer and the corrugated sheath is filled with said insulating composition. I

4. An electric cable according to claim 3, wherein said sheath is a seamless aluminum tube.

5. Anelectric cable according to claim 4. wherein said layer is paper impregnated with said insulating composition. 

1. An electric cable comprising a core having a conductor surrounded by a layer of insulation, an outer metallic sheath surround said core and an insulating composition intermediate say said layer and said outer metallic sheath, said sheath being sinusoidally corrugated with a minimum interior diameter at least 1 millimeter larger than the outside diameter of said layer and with the depth of the corrugations in the range of between 0.5 and 1.5 millimeters and the ratio of said depth to the pitch of said corrugations in the range of 1:25 to 1:15.
 2. An electric cable according to claim 1, wherein said sheath consists of aluminum.
 3. An electric cable according to claim 1, wherein the entire space between said layer and the corrugated sheath is filled with said insulating composition.
 4. An electric cable according to claim 3, wherein said sheath is a seamless aluminum tube.
 5. An electric cable according to claim 4, wherein said layer is paper impregnated with said insulating composition. 